The present invention relates to Moving Target Indicator (MTI) radar systems employing limited transmitted pulse trains or burst waveforms to form single or multiple filters, and more particularly to a circuit for such radars which cancels returns from ambiguous range clutter with less transmitted energy and less time than that required by conventional systems.
MTI radar systems are well known in the art, and have received considerable discussion in the literature. One exemplary general description of MTI radars appears in the book "Introduction to Radar Systems," by Merrill I. Skolnick, McGraw-Hill Book Company, 1980, at Chapter Four. Since the Doppler return from stationary targets have unchanging return amplitudes and range from pulse to pulse, delay-line cancellers are conventionally employed as filters to remove the d-c (unchanging) components of fixed targets and to pass the a-c (changing) components of moving targets. The i-f signal from the receiver (or alternatively a baseband inphase and quadrature representation) is divided between two channels, one a normal i-f channel and the other including a delay line providing a delay equivalent to the inter-pulse-period to the i-f signal. The outputs from the respective channels are then coherently subtracted from one another, and the resultant signal includes only returns from moving targets, since the returns from fixed targets have non-varying phases and amplitudes from pulse to pulse, and are cancelled. This type of processor is known as a single canceller.
To achieve a desired frequency domain performance from the delay-line canceller to cancel particular kinds of clutter, various arrangements including multiple delay lines have been employed. One common configuration, the double canceller, employs two cascaded single canceller circuits, which is equivalent to combining the signal from the present pulse period, the signal from the preceding pulse period with its amplitude weighted by -2, and the signal from two pulse periods previous.
The present invention is concerned with the detection of moving targets by MTI active radars, which process the returns from a plurality of pulses comprising a limited pulse train transmitted at a fixed pulse-repetition-rate (PRF), defining a fixed inter-pulse-period (IPP). The length (in time) of the IPP for a given radar is related to the unambiguous range interval, the maximum target range at which a transmitted pulse may propagate to the target and the target return be reflected back to the radar before the next pulse is transmitted. The unambiguous range interval may be considered the "first range interval." The target (or a stationary object producing clutter returns) could be at a range which is outside that defined by the unambiguous or first range interval such that the return from the target is received in the second IPP following the transmitted pulse. This second IPP following a particular pulse is considered the "second range interval." If the target or stationary object is further away from the radar such that the return is received in the third IPP following a transmitted pulse, then the return is considered to be in the "third range interval." The second and third (or further) range intervals are considered to be ambiguous intervals.
The number of pulses required for MTI operation is a function of the number of returns processed and the number of range intervals over which the processor must be effective in clutter cancellation. The number of processed returns determines the frequency response of the canceller, e.g., the breadth of the clutter rejection null formed by the canceller. Typically the fewer number of returns which are processed, the narrower is the clutter rejection null, centered at zero doppler frequency (stationary object). For many applications, it is desirable to broaden the clutter rejection null, e.g., to cancel returns from slowly moving objects not considered "targets," or to increase the degrees of freedom in tailoring the filter response so as to provide a desired filter characteristic, e.g., sharp filter skirts. To broaden the null or to increase the allowable degrees of freedom, the returns from more transmitted pulses are processed, e.g., as described above with respect to the single and double cancellers.
The number of range intervals over which the processor must be effective in clutter rejection also affects the required number of transmitted pulses. Enough pulses must be transmitted to provide the returns required for the processor operation over the number of range intervals over which effective clutter cancellation is required. The pulses required to be transmitted to meet the range interval clutter cancellation requirements but which are not used in a given processing interval in the processor are known as "fill" pulses. For example, if four pulses are transmitted and the returns from three pulses are processed in a double canceller MTI radar processor, then one pulse is a fill pulse, and the canceller is capable of cancelling clutter in the first and second range intervals. In a conventional MTI system, enough fill pulses are typically transmitted to assure cancellation of clutter at the longest range.
The disadvantages of the use of fill pulses are the required additional time and transmit energy. For pencil beam radars, there is often not enough time to use extra fill pulses and still be able to fill the scan volume with pencil beams in the time allotted. Thus, unless other steps are taken, the MTI processor output will include uncancelled returns from ambiguous range clutter.
It would therefore represent an advance in the art to provide a processor that eliminates returns from ambiguous range clutter with less transmitted energy and time than conventional MTI radar processors.